1. Field of the Invention
The present invention relates to a method of designing a semiconductor integrated circuit incorporating an electrostatic discharge (ESD) protection circuit which protects internal circuits against ESD. More particularly, it relates to a method of designing a semiconductor integrated circuit in which signals are transmitted/received among a plurality of internal circuits operated by different power supply potentials.
2. Description of the Related Art
A plurality of power supply nodes (power supply line or interconnects) isolated for various reasons are used within a semiconductor integrated circuit (hereinafter referred to as an LSI).
The first reason for isolating the power supply nodes lies in the necessity of a plurality of power supply voltages.
An input/output (I/O) circuit unit of the LSI outputs a signal to the output of an LSI chip, and a signal from the outside of the LSI chip is input to the I/O unit. Therefore, a standard for the I/O unit is set to enable the transmission/reception of signals between different LSI chips, and the power supply voltage of the I/O unit is also set. On the other hand, a decreased voltage is effective means for lower power consumption and a higher speed of the LSI, and internal logic circuits other than the I/O unit are typically driven by power supplies at a lower voltage than the I/O unit. It is thus necessary to isolate the power supply nodes.
The second reason for isolating the power supply nodes lies in the measures for noise. When an analog circuit, a radio-frequency (RF) circuit, etc., are incorporated, noise generated by the operation of circuits other that these circuits such as a digital circuit may adversely affect the analog circuit and the RF circuit. Among various conceivable propagation paths of the noise, a path passing through a power supply line can be cited as the main path. In order to prevent an erroneous operation of the circuits, the analog circuit and the RF circuit are isolated from the power supplies of the other circuits.
The third reason for isolating the power supply nodes lies in the measures for reducing the power consumption. In a normal system LSI, there are generally no simultaneous operations of all the circuits within the LSI chip. A technique has therefore been proposed, wherein the LSI internal circuits are classified into function blocks, and no power supply voltage is supplied to the function block during a period at which it does not need to be operated, thereby keeping down the power consumption. In order to achieve this, it is necessary to isolate the power supply nodes on the function block basis.
On the other hand, in a MOS LSI such as a CMOS LSI, an ESD protection circuit is incorporated to protect internal circuits against an overcurrent caused by an ESD. In order to protect against the ESD the LSI in which the power supply nodes are isolated as described above, there has heretofore been a proposal which employs two schemes: a common bus line protection scheme in which all the power supply nodes connected to a power supply terminal are connected to a common node via ESD protection circuits, and a power-to-power protection scheme in which the isolated power supply nodes are connected to one another by an ESD protection circuit (e.g., refer to US2005/0152081).
However, in the case of the power-to-power protection scheme in which the isolated power supply nodes are connected to one another by the ESD protection circuit, an ESD protection circuit can not be inserted between regions which are not adjacent on the chip. Their power supply nodes can be drawn around within the chip so that these power supply nodes may be adjacent to each other, but this is in most cases difficult to achieve due to the limitations of a chip layout.
Furthermore, the insertion of the ESD protection circuit is generally carried out manually after automatic designing in consideration of the flows of signals, which requires experience and time. It has been difficult to clarify all the cases where a signal is transmitted between isolated-power supplied regions which are not adjacent within the chip.
There has thus a demand for the provision of a semiconductor integrated circuit designing method wherein even when a signal is transmitted between isolated-power supplied regions which are not adjacent within a chip, internal circuits can be protected against ESD, and all the circuits within the chip can be protected against the ESD by one ESD protection circuit.